(1) Field of the Invention
The present invention relates to an developing device which performs development of the static latent image formed on the static latent image bearer with a toner as the coloring agent, and in particular, relates to a developing device using a developer as the toner composed of a single component.
(2) Description of the Prior Art
An image forming apparatus using electrophotographic technology, such as a copier, printer, etc. has a developing device which, in order to visualize the static latent image formed on the photoreceptor surface as a latent image bearer, etc., supplies a developer, such as toner or other coloring agents, to the photoreceptor and allows the toner to selectively adhere to the latent image.
In this developing device, the static latent image formed on the photoreceptor is developed and then the developed toner image is transferred to a sheet of paper as the printing material. On passing the transfer step, part of the toner which could not transfer to the sheet remains on the photoreceptor surface. This unnecessary leftover toner is removed from the photoreceptor surface in order to perform a subsequent image forming operation. Therefore, a cleaning device for removal of the leftover toner from the photoreceptor surface is arranged downstream of the transfer station, and the unnecessary toner removed by this cleaning device is collected into the collecting portion in the cleaning device.
With a trend of image forming apparatuses having a developing device of the above type toward being compact, the space for arrangement of the processing devices for image formation around the photoreceptor needs to be reduced. So there has been also a strong demand for a compact developing device.
Instead of using a dual component developer made up of a toner and a magnetic carrier, developing devices which use a mono component developer have been proposed and come into practice. In such a developing device using a mono component toner, there is no longer any necessity to control the toner concentration, and the volume of the developer hopper can be markedly reduced because of the absence of carrier, making it possible to make the developing device compact. In addition, the mono component toner developing device is simple and excellent in maintenance. Specifically, there is no need to replace the developer because of degradation of the developer, especially degradation of the carrier, so that maintenance for its replacement is unnecessary.
Control as to the toner also can be simplified because only the toner needs to be re-supplied while neither toner concentration detection and hence nor its control will be needed. Particularly, in a developing device using a mono component toner, the only thing to be done is to re-supply the toner when it is needed.
For example, as shown in FIG. 4, opposed to a photoreceptor 1 as an image bearer is a developing device 4 for visualizing the static latent image formed on photoreceptor 1. In this developing device 4, a rotatable developing roller 41 is arranged so as to oppose the opening of a developer hopper 40 holding a toner 10, the mono component developer. This developing roller 41 is exposed in part to the opening of developer hopper 40 and arranged in such a manner as to be in contact with photoreceptor 1, for example. This contact area will be the developing area.
Developing roller 41 supports the mono component toner on the surface thereof and conveys it to the developing area opposing photoreceptor 1. After development, the toner which has not been used for development is conveyed into developer hopper 40 and collected therein. The collected toner is once removed from the developing roller 41 surface. For this purpose, a supplying roller 42 is provided in pressing contact with developing roller 41 so as to scrape the toner supported on the developing roller 41 surface. This supplying roller 42 also functions to newly supply the toner to the developing roller 41 surface.
The mono component toner is supplied by supplying roller 42 and adheres to the developing roller 41 surface. In order to regulate the amount of adherence, a regulating means 43 is arranged in pressing contact with the developing roller 41 surface. The toner after having passed through regulating means 43 is regulated as to its amount and reaches the developing area where the toner opposes and is in contact with photoreceptor 1 as stated above. In this area, the toner selectively adheres to the static latent image formed on the photoreceptor 1 surface, thus development is carried out. The developing device having this configuration is disclosed in, for example, Japanese Patent Application Laid-Open Hei 3 No.33776.
In this disclosure, as the above regulating element 43, a plate-like element, instead of a roller configuration shown in FIG. 4, is provided so as to abut developing roller 41. In such a configuration, the life of a plate-like regulating element 43 is determined dependent on the degree of abrasion at the contact portion at which the element abuts developing roller 41. This contact portion comes in sliding contact with developing roller 41 while developing roller 41 rotates, and it is abraded by this contact. When the contact portion has become unusable because of the progress of abrasion, it is understood as the end of life of the regulating element 43. Since in a plate-like regulating element 43, the element is in contact with, and abraded by, the peripheral surface of developing roller, continuously at the unchanged, contact portion, the element becomes badly worn out.
Use of a plate-like element as regulating element 43 makes it possible to provide a simple configuration and reduce the cost, however suffers from marked abrasion at the contact portion with developing roller 41 as stated above. This further leads to liability to cause toner sticking at the contact portion of regulating element 43 with developing roller 41. When the toner sticks to an area, the amount of adherence on developing roller 41 at that area, differing from that in the other areas, becomes extremely low or becomes locally high, bringing about imbalance in the distribution of the toner layer thickness. This results in white stripes or black stripes attributed to the development.
Accordingly, the problem is the shortening of the life of regulating element 43 due to deficiencies of abrasion and toner sticking. This problem has been conventionally solved by providing a roller type regulating element 43 disclosed in Japanese Patent Application Laid-Open Hei 2 No.116872 and shown in FIG. 4. That is, the regulating element is avoided from abutting developing roller 41 at a fixed contact so as to reduce toner sticking and realize a longer life.
The use of regulating element 43 having a roller configuration consequentially stabilizes the amount of toner adherence on developing roller 41 and beneficially prevents unevenness in the amount of charge on the toner, providing the effect of stabilizing the development. However, the problem of toner sticking still occurs even using such regulating element 43 of a roller configuration. For example, when the development with the developing roller has been completed, the toner deposited on the developing roller is pressed by the regulating roller that is pressed against the developing roller as the toner moves together with the developer roller. During this period this, i.e., during development, the developing roller has a bias voltage applied thereto which causes the toner to be attracted toward the developing roller surface. Thus, any combination of these may cause toner to stick on the developing roller.
In the developing roller configured as in FIG. 4, the same phenomenon as in the position of regulating element will occur at the position of supplying roller 42. Actually, supplying roller 42 also is rotated with it also being abutted against developing roller 41 so as to supply the toner to developing roller 41 whilst scraping the toner after development. When developing roller 41 stops as the development is completed, supplying roller 42 is also stopped. Therefore, the toner held at that nip between the rollers may stick to the developing roller 41 surface or the supplying roller 42 surface.
Once the toner has stuck to the developing roller 41 surface, not only the development conditions change, presenting deficiency in development but also the toner is unevenly applied on the developing roller 41 surface, forming stripes, white voids during development thus degrading the image quality.
For countermeasures against this, Japanese Patent Application Laid-Open Hei 5 No.113714, for example, discloses a configuration in which the developing roller is rotated in the reverse direction before the start of the developing action of the developing roller or after development, to prevent sticking of the toner to the developing roller etc. As the developing roller is rotated in the normal direction to perform development after this control, the toner layer deposited on the developing roller is made uniform in thickness to achieve beneficial development.
The above reverse rotation of the developing roller is effective in preventing toner adherence and sticking to the developing roller, but if it is done before development, this naturally delays the start of development or the start of image formation. Therefore, it is preferable to remove the toner adhering on the developing roller by reversing the developing roller after development and have the apparatus ready for a next operation with toner sticking prevented.
However, trying to remove the unnecessary toner adhering on the developing roller at the position of the regulating element by reversing the developing roller cannot present good enough scraping effect. Further, in the case where the developing roller is reversed, there is a risk that the toner having been scraped off by the regulating element cannot be collected inside the developing hopper and might scatter outside the developing hopper.
Moreover, in a configuration where contact development is employed or the developing roller is set in contact with the photoreceptor as a static latent image bearer, if the scheme of reversing the developing roller is adopted, frictional force etc. acting on the photoreceptor causes rather severe damage to the photoreceptor. Actually, in the system where the damage to the photoreceptor due to the rotation during the developing action is minimized, if the rotation is reversed, abrasion on and damage to the photoreceptor surface are increased, shortening the life of the photoreceptor. Further, since an extra driving mechanism is needed to rotate the developing roller in the reverse direction, this increases the cost from both the structural and control aspects.
In this way, the configuration where the developing roller is rotated in reverse is effective when it is applied to a developing unit with its developing roller kept out of contact with the photoreceptor, while it is not very effective if it is applied to a developing unit with its developing roller kept in contact with the photoreceptor.
In the aforementioned developing unit disclosed in Japanese Patent Application Laid-Open Hei 2 No.116872, a cleaning blade with its distal end abutted against the regulating roller is provided so as to scrape (clean) the toner adhering on the regulating roller, whereby the toner layer deposited on the developing roller is made uniform. Since this configuration makes it possible to remove the toner adhering to the regulating roller by means of the blade, the problem of toner sticking and others can be eliminated to a certain degree.
However, the above configuration where the regulating element is provided in a roller form as regulating roller 4 with a cleaning blade for scraping the adhering toner, needs the pressing condition of the cleaning blade correctly setting, etc., requiring very burdensome adjustment and tasks. In particular, the increase in contact pressure in order to enhance the cleaning effect results in the risk of the regulating roller surface being damaged, while no cleaning effect can be expected if the contact pressure is too small.
Thus, since the cleaning blade is provided to scrape the toner remaining on the regulating roller surface, this configuration makes it possible to avoid the toner from being pressed between the developing roller and the regulating roller at the end of development and hence it is possible to inhibit such toner from sticking in a limited degree, but the adjustment task becomes very burdensome as stated above.
In addition,.even if the regulating roller surface is cleaned and the toner is regulated so as to be uniform by abutting the cleaned, regulating roller against the developing roller, the developing roller is supplied with the toner for development by means of the supplying roller. That is, the amount of the supplied toner is limited by the regulating roller but the toner inevitably exists between the regulating roller and the developing roller when the rollers are stopped at the end of development. Therefore, if the machine is left for a long time with the toner squeezed therebetween, it is impossible to effectively solve the toner sticking and other problems.
Moreover, the toner sticking problem occurs not only between the regulating roller and the developing roller, but also occurs between developing roller and the supplying roller. In conclusion, prevention against toner sticking cannot be essentially eliminated even if the above configuration where a cleaning blade is provided for the regulating roller, as disclosed in the above publication, is employed as stated above.
It is therefore an object of the present invention to provide a developing unit using a mono component toner, having a supplying roller for supplying the toner to the developing roller, which effectively prevents toner sticking to the developing roller and other components without causing any damage to the static latent image bearer, any toner scattering and any undesirable effects.
It is another object of the present invention to provide a suitable means for eliminating the toner sticking problem with developing units, not limited to those using a developing roller kept out of contact with the static latent image bearer but also applicable to those using a contact type developing roller.
It is a further object of the present invention to a developing unit in which rotations of the regulating roller and supplying roller after development are controlled so as to remove the toner existing between the developing roller and these rollers, whereby the cause of toner sticking is eliminated.
In order to achieve the above objects, the present invention is configured as follows:
In accordance with the first aspect of the present invention, a mono component toner developing device, includes: a developing roller supporting and conveying the mono component toner to a developing area opposing a static latent image bearer; a supplying roller for supplying the toner to the developing roller, as the preparative stage for facilitating the developing roller to convey the toner to the developing area; and a controller which controls the bias voltages to the developing roller and supplying roller so that the potential difference therebetween during development creates an electric field that will cause the toner to move toward the developing roller side, switches the bias voltages, at the timing when the static latent image on the static latent image bearer is completely developed passing through the developing area, so that the polarity of the potential difference between the rollers becomes reversed compared to that during development, and keeps the potential difference during a predetermined period of time (t1) up to the rotation stop of the developing roller for terminating the developing operation.
In accordance with the second aspect of the present invention, the mono component toner developing device having the above first feature is characterized in that the controller switches the bias voltage applied to the supplying roller from the voltage during development to such a voltage that the potential difference between the supplying roller and developing roller becomes reversed compared to that during development.
In accordance with the third aspect of the present invention, the mono component toner developing device having the above first feature is characterized in that the controller stops rotation of the developing roller after a lapse of the predetermined time (t1), keeps up the drive of the supplying roller for a fixed period of time whilst maintaining the reversed state of the potential difference between the supplying roller and developing roller, and stops all the rollers and their voltage application after a lapse of the fixed time.
In accordance with the fourth aspect of the present invention, the mono component toner developing device having the above first feature is characterized in that the potential difference between the developing roller and supplying roller during development and that after the switching control following the development are set to be lower than the discharge starting voltage.
In accordance with the fifth aspect of the present invention, the mono component toner developing device having the above first feature is characterized in that the predetermined time (t1) is set so as to at least allow the developing roller to rotate more than one revolution.
In accordance with the sixth aspect of the present invention, a mono component toner developing device, includes: a developing member (roller) supporting and conveying the mono component toner to a developing area opposing a static latent image bearer; a supplying roller for supplying the toner to the developing roller, as the preparative stage for facilitating the developing roller to convey the toner to the developing area; a regulating roller for limiting the toner supplied to the developing roller to a constant amount before the toner reaches the developing area; a cleaning roller disposed in contact with the supplying roller and the regulating roller; and a controller which stops the developing member after the static latent image on the static latent image bearer has been developed whilst keeping on the rotation of supplying roller, regulating roller and cleaning roller for a predetermined period of time, and switches the levels of the bias voltages, in time with the stop control of the developing member so that the directions of the electric fields acting on the toner, formed at least between the developing member and the supplying roller and between the developing member and the regulating roller after switching are opposite to those formed during development.
In accordance with the seventh aspect of the present invention, the mono component toner developing device having the above sixth feature is characterized in that the controller switches the level of the bias voltage applied to the cleaning roller so that the directions of the electric fields acting on the toner, formed between the supplying roller and the cleaning roller and between the regulating roller and the cleaning roller after switching are opposite to those formed during development.
In accordance with the eighth aspect of the present invention, the mono component toner developing device having the above sixth or seventh feature is characterized in that upon the switching control of the bias voltages, the controller regulates the potential differences to be equal to or lower than the discharge starting voltage.
In accordance with the ninth aspect of the present invention, the mono component toner developing device having the above sixth feature is characterized in that the controller stops rotation of the supplying roller and regulating roller after a lapse of the predetermined period of time and keeps on controlling and driving only the cleaning roller for a predetermined period of time.
In accordance with the tenth aspect of the present invention, the mono component toner developing device having the above ninth feature is characterized in that the controller turns off the application of voltages to the supplying roller and the regulating roller, or the application of voltages to the above two rollers and the cleaning roller, after a lapse of the predetermined period of time.
In accordance with the eleventh aspect of the present invention, the mono component toner developing device having the above sixth feature is characterized in that the predetermined time is set so as to at least allow the supplying roller, regulating roller and cleaning roller to rotate more than one revolution.
According to the developing device having the above first configuration, during the predetermined period of time (t1) before the completion of the developing operation, the potential difference between the developing roller and the supplying roller after switching is controlled so as to be opposite to that during development. That is, when the static latent image has been developed passing through the developing area, the potential difference is controlled during the predetermined period (t1) after the development so as to be reversed compared to that during development. This generates an electric field opposite to that during development, so as to cause the toner to transfer from the developing roller side to the supplying roller side instead of the toner being supplied from the supplying roller to the developing roller. Therefore, toner sticking to the developing roller can be inhibited. In addition, the supplying roller as it rotates without supplying the toner can scrape the toner adhering on the developing roller, enhancing the prevention effect against toner sticking to the developing roller.
In this case, the developing roller is kept on rotating as is and the supplying roller is also kept on rotating as is. So deficiencies which would occur if the developing roller was rotated in reverse will not occur. Thus, it is possible to inhibit occurrence of abrasion and damage to the photoreceptor as a static latent image bearer such as a photoreceptor having an organic photoconductive film (OPC).
Next, according to the above second aspect, in the developing device having the above first configuration, the controller switches the bias voltage applied to the supplying roller from the voltage during development to such a voltage that the potential difference between the supplying roller and developing roller becomes reversed compared to that during development. Therefore, the developing bias voltage applied to the developing roller can be kept as is. As a result, it is possible to prevent any unnecessary toner from adhering to the non-image area due to the developing roller rotation after development, still using a simple switching control configuration of switching the bias voltages.
Further, according to the above third aspect, in the developing device having the above first configuration, the controller stops rotation of the developing roller after a lapse of the predetermined time (t1), keeps up the drive of the supplying roller for a fixed period of time whilst maintaining the reversed state of the potential difference between the supplying roller and developing roller, and stops all the rollers and their voltage application after a lapse of the fixed time. Thus, since rotation of the supplying roller is kept on, the toner adhering on the supplying roller can be efficiently removed by the stationary developing roller. Therefore, this configuration inhibits toner adherence to the supplying roller and hence can inhibit sticking to the supplying roller, thus preventing toner sticking to the developing roller in an efficiently manner.
Moreover, according to the above fourth configuration, in the developing device having the above first configuration, the potential difference between the developing roller and supplying roller during development and that after the switching control following the development are set to be lower than the discharge starting voltage. This configuration can prevent occurrence of electric discharge which might occur during switching control etc. In particular, if discharge occurred, the toner would be heated and fused and might stick to the developing roller and supplying roller. This configuration can prevent occurrence of such events.
According to the above fifth configuration, in the developing device having the above first configuration, the predetermined time (t1) is set so as to at least allow the developing roller to rotate more than one revolution. Therefore, the entire peripheral surface of the developing roller can removed of the toner. For example, when the developing roller is adapted to rotate five or six revolutions, it is possible to almost exclude the toner from the nip between the developing roller and the supplying roller. Further, when the supplying roller is kept on rotating for a fixed period of time (t2) while the developing roller is stopped when the developing operation is completed, the fixed period of time should be set so as to at least allow the supplying roller to rotate more than one revolution. This enables removal of the toner from the entire surface of the supplying roller and resultantly enhances the prevention effect against toner sticking.
Next, according to the developing device having the above sixth configuration, the developing member is stopped when the developing operation is ended. In this case, the static latent image bearer is also stopped and the application of developing bias voltage during development is also turned off at the same time. This arrangement prevents the toner from transferring to the photoreceptor side. On the other hand, the supplying and regulating rollers are kept on rotating for a predetermined period of time and the levels of the bias voltages are shifted from those applied during development. Illustratively, during the developing operation, the bias voltages are applied so that the electric fields will cause the toner to move in the directions indicated by the arrows shown in FIG. 12A. In particular, in order to cause the toner to transfer to the developing member (41), the bias voltages applied to the supplying roller (42) and regulating roller (43) relative to the developing bias voltage are set so that the directions of the forces acting on the toner by the electric fields are oriented as indicated by the arrows. When the developing operation is completed and rotation of the developing member (41) is stopped, the levels of the bias voltages are switched from those during development to the different levels so that the electric fields will cause the toner to move to the supplying roller 42 and regulating roller 43 sides. In this way since the supplying roller and regulating roller are kept on rotating with the attractive and repulsive forces during development weakened, the toner adhering on the developing member is effectively removed. That is, the toner residing between the developing member and supplying roller and between the developing member and regulating roller can be positively removed and hence the toner to be trapped is excluded, thus eliminating the problem of toner sticking and making it possible to keep up the stable development in the long term.
Next, according to the above seventh aspect, in the developing device having the above sixth configuration, the controller switches the level of the bias voltage applied to the cleaning roller so that the directions of the electric fields acting on the toner, formed between the supplying roller and the cleaning roller and between the regulating roller and the cleaning roller after switching are opposite to those formed during development. Therefore, this configuration enhances the toner removal effect of excluding the toner from the nip between the cleaning roller (45) and supplying roller (42) and the nip between the cleaning roller (45) and regulating roller (43) as shown in FIG. 12B so as to eliminate the factor itself that will cause toner sticking, thus making it possible to keep up the stable development in the long term.
Further, according to the above eighth aspect, in the developing device having the above sixth or seventh configuration, upon the switching control of the bias voltages, the controller regulates the potential differences to be equal to or lower than the discharge starting voltage. Therefore, it is possible to prevent the toner from sticking due to heat which would be generated by the electric discharge current. That is, since the bias voltages is set so as not to cause discharge upon the switching control of the bias voltages when the developing operation is ended, it is possible to eliminate deficiencies due to electric discharge.
Moreover, according to the above ninth aspect, since in the developing device having the above sixth configuration, the controller stops rotation of the supplying roller and regulating roller after a lapse of the predetermined period of time and keeps on controlling and driving only the cleaning roller for a predetermined period of time, the toner adhering on the supplying roller and regulating roller which are both standing still can be removed by the cleaning roller. Thus, the toner residing at the areas in contact with the cleaning roller can be removed to thereby eliminate the factor of causing toner sticking and hence eliminate the deficiencies thereby.
Furthermore, according to the above tenth aspect, since in the developing device having the above sixth configuration, the controller turns off the application of voltages to the supplying roller and the regulating roller, or the application of voltages to the above two rollers and the cleaning roller, after a lapse of the predetermined period of time, it is possible to prevent the toner from fusing and sticking due to Joule heat which would be generated by micro current flowing between the rollers. In particular, the adhering toner when rotation of the supplying roller and regulating roller is stopped can be prevented from fusing from the aforementioned heat.
Finally, according to the above seventh aspect, since in the developing device having the above sixth configuration, the predetermined time is set so as to at least allow the supplying roller, regulating roller and cleaning roller to rotate more than one revolution, it is possible to remove the adhering toner from the entire peripheral surfaces of the rollers and hence completely exclude the toner remaining between the rollers. Therefore, it is possible to eliminate the cause of toner sticking. Thus, it is possible to eliminate development failures due to toner sticking and keep up the stable development in the long term.